The bombardment of workpieces with molecular, atomic or subatomic particles has been used for abrading the surfaces of workpieces, for oxidizing surface contaminants or for sterilizing the surface by killing organisms. In such a system an ionizable gas is subjected to an electric or electromagnetic field to free and accelerate electrons. Free electrons gain energy from the imposed electric field and impart this energy through collision with electrically neutral gas molecules to ionize the gas molecules when they collide with them. The collisions produce an ion and further electrons which are then accelerated in opposite directions by the field. A workpiece positioned in the plasma, generated in this manner, is bombarded by these particles. Particles, for example may include oxygen ions, thereby enhancing the probability of oxidation of surface contaminants. Also, other gases, such as aldehydes, have been used for sterilization. Others have used argon or nitrogen to effect a surface abrasion.
One example of such a system is shown in U.S. Pat. No. 4,207,286 in which a coil, wound about a low pressure chamber is connected to a radio frequency signal generator to subject the chamber to an electromagnetic, alternating field at RF frequencies. Such a glow discharge induced by an RF electromagnetic field has also been used for the deposition of a film upon the surface of a workpiece, such as illustrated in U.S. Pat. No. 4,632,842. Yet other plasma/glow discharge treatment systems are illustrated in U.S. Pat. Nos. 4,348,357 and 4,656,083.
In an article entitled "Implant Surface Preparation" by Baier and Meyer, which appeared in Volume 3, No. 1, 1988 issue of The International Journal Of Oral & Maxillofacial Implants, the authors review various cleaning methods and give the reasons that they are necessary and conclude that glow discharge treatment shows great promise for prosthetic dental devices.
In addition to bombardment with particles from a glow discharge plasma, others have proposed the irradiation of workpieces with ultra violet light, such as is illustrated in U.S. Pat. No. 3,864,081. Ultraviolet systems often use mercury vapor lamps as the most effective way to sterilize.
In U.S. Pat. No. 3,876,373 workpieces are sterilized using reactive gases at low energies, creating a plasma discharge at sonic frequencies. This patent not only uses reactive gases but also seeks a resonant effect upon the living microorganisms. The difficulty with the low energy use of reactive gases at sonic frequencies is that the biological effectiveness of utilizing specific resonant frequencies to destroy microorganisms is not sufficiently proven or reliable and the use of reactive gases imposes dangers to human operators and problems with safely handling these reactive gases.
In the typical radio frequency apparatus a coil is wrapped around the low pressure chamber and a current is applied to the coil which alternates at a radio frequency. The current creates an electromagnetic field inside the chamber which ionizes the gas and creates the electron/ion plasma. This causes electrons to flow in a circular pattern with positive ions and electrons flowing in opposite directions. Each time the field reverses the direction of these particles reverses. The field pattern of these particles has the greatest particle motion or energy near the outer wall of the chamber.
Such RF units raise problems, including impedance matching, frequency stability, and the generation of a non-uniform plasma flow within the chamber. Impedance mismatching occurs when a conducting target placed in a chamber is sufficiently large to effect the RF currents. While automatic matching circuits can alleviate this problem, the mismatch can be so significant as to cause either automatic shut-down of the equipment or to burn out the power circuitry. Frequency stability is important. It is regulated by a governmental agency to prevent interference with other equipment operating at radio frequencies.
Because they operate at radio frequencies, the electromagnetic energy applied to the coil which surrounds the chamber is partially radiated into the atmosphere. This means that the voltage and current applied to the coil do not bear a sufficiently accurately known or constant relationship to the particle currents within the plasma to enable them to be used to control the process. Additionally, these emissions cause shielding and radio emission problems which can interfere with other uses of the frequency spectrum.
Furthermore, in coil driven RF systems the plasma is formed in a thin circular pattern within the chamber, leaving a relatively large, central cold space in which there is relatively little particle motion with relatively low energy particles to collide with the workpiece being treated. Thus, in the RF systems the effective work area is confined to a thin, cylindrical, circumferential layer requiring that the workpiece not extend inwardly beyond this layer, therefore limiting the size of the chamber and the workpieces.
Thus, in summary, the prior art teaches that, in the use of a glow discharge plasma for the treatment of workpieces, one uses sonic frequencies to effect the microbiological organisms and uses radio frequencies in order to eliminate electrodes from the chamber. It also teaches the use of biologically active gases. However, since biologically active gases pose handling problems and the use of RF energy poses control problems, there is therefore a need for a cleaning technique and system which overcomes these problems to provide for the effective cleaning of the workpieces under suitable control in order to optimize the cleaning effectiveness and minimize the time and equipment required for that effective cleaning.